This invention relates to photographic exposure control systems, and in particular to exposure control systems for appropriately exposing both the main object and the remainder of the overall scene being photographed, especially when flash illumination is used.
Hitherto known mechanisms have compensated for insufficient brightness of an object being photographed by momentarily illuminating the object, or subject, with a flash illuminant such as a xenon discharge lamp or flash bulb.
However, to use the light from a flash apparatus, it is necessary to compute the value of the diaphragm aperture in a camera by means of a formula based upon the amount of light from the flash illuminant. With existing systems, the sensitivity in ASA units of the film being used and the distance to the object being photographed is entered into the computation before the iris of the camera is adjusted to the resulting value. This arrangement selects the distance from the camera to the main object from all the other objects being photographed, for adjusting the diaphragm aperture of the camera. This makes the exposure appropriate for the portion of the scene containing the main object.
However, even when the main object to be flash illuminated and photographed may be dim, from the practical point of view its brightness is very rarely zero. The natural or available light affords it some brightness. Thus the additional illumination provided by the flash unit on the basis of the formula involving film sensitivity, flash intensity, and distance to the main object may result in an overexposed frame. Nevertheless, most objects to be photographed with flash are not significantly overexposed because the natural or available light is generally considered.
When the brightness of the main object in an overall scene to be photographed exceeds a predetermined level, such as when flash is used in daylight, the aperture setting based upon the aforementioned flash computation is likely to create an overexposed picture.
U.S. Pat. No. 3,464,332 discloses facilities for solving some of these defects by utilizing a flash bulb having a comparatively long illumination period and entering into the diaphragm computation both the anticipated light level of the flash illuminated main object and the distance to the main object. This establishes a coarse exposure for the main object. Subsequently the diaphragm is varied by coupling it with a diaphragm disposed in front of a photocell that controls the shutter time constant circuit in response to the light received by the photocell so as to vary the shutter time and perform a fine adjustment on the exposure system.
In such a system when the main object is dim, an appropriate exposure is obtained for the portion of the overall scene containing the main object through this preset diaphragm, by closing the shutter after illumination of the flash bulb is completed. However, when the portion of the overall scene containing the main object has been illuminated by available or natural light, the photocell becomes sensitive to the illumination of the flash bulb and causes the shutter period to be shortened in order to prevent overexposure. For instance, the shutter is adapted to close before the period of flash bulb illumination is completed so as to eliminate any unnecessary exposure when the brightness of the main object is quite high.
The term "available light" as used herein conforms to the term generally used by photographers when speaking of "available light photography." In the broad sense it includes both natural and artificial illumination that may happen to be present when a photograph is being taken, but which is not specifically provided by the photographer for photographic purposes. Thus it may include the artificial illumination normally provided indoors by conventional incandescent or fluorescent household lamps and any illumination passing through the window from outside. In a specific sense the term is used for indoor photography with such light that happens to be immediately available.
In the nature of things a photograph may be divided into two parts, namely the portion containing the main object or the essential part of the main object being photographed, e.g., the subject, and the background or remainder of the scene. Thus setting the exposure only for the subject of the photograph does not necessarily create an appropriate exposure for the background or an appropriate average exposure for the whole picture.
The foregoing defects are caused by the fact that a photograph is composed of the portion containing the main object and the remainder of the scene, e.g. the subject and the background, and the distance to the camera of each and the brightness of each is likely to be different. To explain this in more detail, finely adjusting the exposure to be appropriate for the portion of the photograph containing the main object from among all those objects to be photographed does not appropriately expose the remaining portions of the scene. Thus the resulting picture is adjusted for appropriate exposure only for the portion containing the main object. Usually such a procedure cannot help but improperly expose the background or remaining portions of the scene.
A striking example of the foregoing defect occurs in pictures involving a back-lighted person at the seaside, namely where both the sunshine and the sea are behind the person constituting the main object. In such a case, the subject is back-lighted. The brightness of the background, namely the seaside, is much higher than that of the person. The brightness of the person is apt to be insufficient. Therefore a diaphragm aperture is set to provide appropriate exposure for the subject with flash illumination. The procedure is capable of properly exposing the main object illuminated by flash. On the other hand, the exposure is improper for the background. Thus the overall photograph is inappropriately exposed.
An object of the present invention is to eliminate the aforementioned drawbacks.
Another object of the invention is to provide an exposure control system for flash photography which produces an appropriate exposure of both the subject and the background being photographed.
Yet another object of the invention is to provide an exposure control system usable with a flash apparatus and involving both the iris and the shutter speed.